Frequency Tunable Wire Lasers

ABSTRACT

The present invention provides frequency tunable solid-state radiation-generating devices, such as lasers and amplifiers, whose active medium has a size in at least one transverse dimension (e.g., its width) that is much smaller than the wavelength of radiation generated and/or amplified within the active medium. In such devices, a fraction of radiation travels as an evanescent propagating mode outside the active medium. It has been discovered that in such devices the radiation frequency can be tuned by the interaction of a tuning mechanism with the propagating evanescent mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional ApplicationNo. 61/257,726 filed on Nov. 3, 2009 and entitled “Tunable Wire Lasers,”which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant NNX07A199Gby NASA, Grant FA9550-06-1-0462 by AFOSR, and Grant ECCS0853470 by NSF.The government has certain rights in the invention.

BACKGROUND

The present invention relates generally to frequency tunablesemiconductor radiation-generating devices, such as lasers (e.g.,tunable wire lasers), as well as methods for tuning frequency of suchdevices.

Frequency tunable lasers are employed in a variety of applications, suchas spectroscopy, imaging (e.g., coherent optical tomography (OCT)), andsensing applications among others. Conventional techniques for tuningthe frequency of a semiconductor laser can include, e.g., changing therefractive index of the lasing medium (e.g., via a change in itstemperature) and/or changing the longitudinal mode profile of the laserradiation (e.g., via a change in the cavity length in external-cavitylasers).

The conventional laser frequency tuning techniques, however, suffer froma number of shortcomings. For example, the conventional frequency tuningmethods are difficult to implement in lasers in which the wavelength ofthe laser radiation in the active medium is much larger than across-sectional dimension of the active medium. For example, theradiation wavelength of a laser operating in the terahertz region of theelectromagnetic spectrum (e.g., a radiation wavelength in a range ofabout 30 microns to about 300 microns) can be significantly larger thana cross-sectional dimension of the active medium. The difficulties intuning the frequency of terahertz lasers are evident in that continuousfrequency tuning over a broad frequency range (e.g., greater than 100GHz) using an external-cavity grating has yet to be achieved, andelectrical tuning, e.g., by changing the refractive index due totemperature or due to a cavity-pulling effect, produces a relativelysmall fractional tuning (<1%).

Hence, there is a need for enhanced methods for tuning the frequency ofsemiconductor radiation-generating devices (e.g., terahertz lasers).There is also a need for enhanced semiconductor frequency tunableradiation-generating devices operating at wavelengths that aresignificantly greater than the cross-sectional dimension(s) of theiractive medium.

SUMMARY

In one aspect, the present invention provides a frequency tunable wirelaser that includes a lasing medium for generating laser radiationhaving a transverse mode profile in the lasing medium and a frequency.The tunable wire laser can also include a frequency tuning mechanismexternal to the lasing medium for changing the transverse mode profileof the laser radiation in the lasing medium so as to adjust the laserfrequency. In some embodiments, a transverse cross sectional dimensionof the lasing medium can be at least about 5 times, and in some cases atleast about 10 times, less than a wavelength of the laser radiation inthat medium.

In the above wire laser, a fraction of the laser radiation field of thetransverse mode profile can extend outside the lasing medium andpropagate along the lasing medium as a propagating evanescent wave. Ithas been discovered that the frequency of the laser radiation can betuned by adjusting the amount of the laser energy that is contained inthis propagating evanescent wave. For example, the tuning mechanism caninteract with the propagating evanescent radiation so as to modulate thetransverse mode profile (e.g., increase or decrease the degree ofconfinement of the traverse mode profile within the lasing medium),thereby adjusting the laser frequency.

In some embodiments, the frequency tuning mechanism can include a tuningelement movable relative to the lasing medium, which can be configuredto tune the laser frequency without causing a substantial change, or insome cases any change, in a longitudinal mode profile of the laserradiation. An actuator can be coupled to the tuning element for movingit so as to change the laser frequency as a function of distance of thetuning element relative to the lasing medium. The tuning element can beformed of any suitable material, for example a metal, such as gold,silver, copper and aluminum or a dielectric, e.g., a semiconductor suchas silicon, germanium, and compound semiconductors; or plastics.

In a related aspect, the frequency tuning mechanism can be configured toallow tuning the laser frequency about a central frequency over a rangeof about 30% of the central frequency. In many embodiments, thefrequency tuning mechanism can be configured to provide continuoustuning of the laser frequency over a selected frequency range.

The teachings of the invention are applicable to wire lasers operatingin any frequency range. By way of example, the laser can operate in afrequency range of about 1 THz to about 10 THz.

In some embodiments, the wire laser can include a double metal waveguidecoupled to the lasing medium so as to provide mode confinement of thelaser radiation. For example, the transverse mode profile that isemployed for tuning the laser frequency can extend along the width ofthe lasing medium of the laser, and the double metal waveguide canprovide mode confinement of the laser direction along an orthogonaltransverse direction (e.g., along the height of the lasing medium).

In some embodiments, the lasing medium can include a distributedfeedback (DFB) grating that is configured to facilitate single modelasing. While the DFB can have many configurations, the DFB grating caninclude, for example, an asymmetric distributed feedback corrugationstructure such that the lasing medium includes a corrugated side and asubstantially flat side. In such embodiments, the frequency tuningmechanism can include a tuning element that is movable relative to theflat side. By way of example, the tuning element can be configured tomove along a direction substantially orthogonal to the flat side of thelasing medium so as to vary its distance relative to the lasing mediumfor tuning the laser frequency. In some embodiments, the tuning elementcan be in the form of a block having a flat side that is substantiallyparallel to the flat side of the lasing medium and the laser frequencychanges as a function of a distance between the flat side of the tuningblock and the flat side of the lasing medium. In some embodiments, theflat side of the tuning block and that of the lasing medium are parallelrelative to each other with an accuracy of better than about 1 degree.By way of example, in some embodiments, the tuning of the laserfrequency can be achieved by varying the distance between the flatsurface of the tuning block and that of the lasing medium within a rangeof about 0.5 microns and 20 microns for tunable laser operating at THzfrequencies.

In some embodiments, e.g., those in which the tuning element is formedof a metal, the laser frequency increases as the distance between thetuning element (e.g., a metalized surface of the tuning element) and thelasing medium (e.g., a flat side surface of the lasing medium)decreases. In other embodiments, e.g., those in which the tuning elementis formed of a dielectric material, the laser frequency decreases as thedistance between the tuning element and the lasing medium decreases.

In some embodiments, the tuning element can be implemented as amicroelectromechanical systems (MEMS) device, which can be coupled to adevice wafer on which a wire laser is disposed. For example, such a MEMSdevice can include two spring stages that are mechanically coupled toone another, where one stage can be pushed via an external actuatortoward the second stage to cause movement thereof. The movement of thesecond stage can in turn change, e.g., the distance between a sidesurface thereof (e.g., a metal coated side surface) relative to thelasing medium of the wire laser so as to tune the frequency of the laserradiation in a manner discussed above. In some cases, the second stageis configured to be stiffer than the first stage, for example, thesprings associated with the second stage can be several times (e.g., 2or more times) thicker than those of the first stage.

A variety of actuators can be employed for moving the tuning elementrelative to the lasing medium. For example, the actuator can be amechanical, an electronic, or an electromechanical device. Some examplesinclude, without limitation, a differential micrometer and apiezoelectric actuator. In some cases in which the tuning element isimplemented as a MEMS device, the actuator can be integrated in the MEMSdevice. For example, it can be implemented as a comb-drive mechanism.

In some embodiments, the wire laser can be optically coupled to anamplifier, e.g., a semiconductor optical amplifier (SOA), such that theamplifier receives the laser radiation emitted by the laser through aninput facet (port) thereof. The amplifier amplifies the receivedradiation, which then leaves the amplifier through an output facet(port) thereof. In some embodiments, the amplifier has a taperedconfiguration and its output facet can be anti-reflection (AR) coatedand tilted relative to an output facet of the lasing medium to preventdirect reflection of the radiation back to the lasing medium. Further,in some embodiments a radiation-absorbing layer can be placed on a topsurface of the amplifier to dampen any residual reflection from itsoutput facet. Moreover, in some embodiments a lens, such as a siliconhemispherical lens, can be coupled to the output facet of the amplifierto increase the radiation output power and/or to reduce the divergenceof the output beam.

In other aspects, a tunable laser is provided that includes asemiconductor lasing medium for generating laser radiation having a modecharacterized by a transverse distribution of the laser radiation fieldsuch that a fraction of the radiation field extends outside the lasingmedium as the laser radiation propagates along the lasing medium. Thetunable laser can further include a tuning element external to thelasing medium. The tuning element can be movable relative to the lasingmedium for interacting with the radiation field extending outside thelasing medium so as to change the frequency of the laser radiation.

In some embodiments, the lasing medium can have at least one transversecross-sectional dimension that is at least about 5 times, or at leastabout 10 times, less than a wavelength of the laser radiation in thelasing medium. In some implementations, the tunable element can beconfigured to reduce the fraction of the laser radiation field extendingoutside the laser medium as its distance decreases relative to thelasing medium so as to cause an increase in the laser radiationfrequency. In other implementations, the tuning element can beconfigured to enhance the fraction of the laser radiation fieldextending outside the laser medium as its distance decreases relative tothe lasing medium so as to cause a decrease in the laser radiationfrequency. An actuator can be coupled to the movable tuning element formoving it relative to the lasing medium.

In general, the transverse distribution of the laser radiation can becharacterized by a transverse wave vector, and the longitudinaldistribution of the laser radiation within the lasing medium can becharacterized by a longitudinal wave vector. In some cases, the laserfrequency and the transverse and longitudinal wave vectors are relatedby the following relation:

k _(z) ² +k _(⊥) ²=ω²μ∈

in which k_(z) denotes the longitudinal wave vector, k_(⊥) denotes thetransverse wave vector, μ denotes magnetic permeability of the lasingmedium, ∈ denotes dielectric constant of the lasing medium, and ωdenotes angular frequency of the laser radiation, which in turn isdefined as:

ω=2πf,

in which f denotes the laser frequency. The tuning element can changethe transverse wave vector k_(⊥) so as to change the frequency of thelaser radiation. In many embodiments, the change of the transverse wavevector can be achieved without changing the other parameters, such asthe longitudinal wave vector (k_(z)) or the dielectric constant (∈).

In a related aspect, a method for tuning a wire laser is provided thatincludes adjusting a transverse mode of laser radiation generated by alasing medium of the laser so as to change the frequency of the laserradiation.

In a related aspect, the step of adjusting the transverse mode can beperformed without causing a substantial change, or in many cases anychange, in the longitudinal mode of the laser radiation. By way ofexample, the step of adjusting the transverse mode can include changinga transverse distance of an external tuning element relative to thelasing medium. The external tuning element can be formed of any suitablematerial, such as a metal and/or a semiconductor. In some embodiments,the wire laser can be configured to generate laser radiation with afrequency in a range of about 1 THz to about 10 THz.

In other aspects, a tunable terahertz laser is provided that includes asemiconductor lasing medium for generating laser radiation with afrequency in a range of about 1 THz to about 10 THz. The lasing mediumcan have a width that is at least about 5 times, and in some cases atleast about 10 times, less than a wavelength of the laser radiation.Further, the laser can include a frequency tuning mechanism having atuning element movable relative to the lasing medium for changing atransverse mode profile of the laser radiation so as to tune the laserfrequency.

In another aspect, a semiconductor wire laser is provided that includesa lasing medium for generating laser radiation propagating along thelasing medium and characterized by a frequency. The laser radiation canhave a profile characterized by a longitudinal wave vector and atransverse wave vector. Furthermore, a frequency tuning mechanism isalso provided for tuning the laser frequency by adjusting the transversewave vector without causing a substantial, or in many cases any, changein the longitudinal wave vector.

In other aspects, the present invention provides incoherent radiationsources, such as amplified spontaneous emission (ASE) sources, thatinclude an active medium having at least one transverse dimension with asize that is at least about 5 times, or at least about 10 times, lessthan the wavelength of the radiation generated by the device in theactive medium such that a portion of the radiation propagates along theactive medium as a propagating evanescent wave. A frequency tuningmechanism external to the active medium can interact with thepropagating evanescent wave in order to adjust a transverse mode profileof the radiation in the active medium, thereby changing the frequency ofthe radiation. For example, the frequency tuning mechanism can include aplunger movable relative to a side surface of the active medium so as tomodulate the confinement of the transverse mode profile within theactive medium as a function of a change in its distance relative to theactive medium. By way of example, the plunger can comprise a metalportion that can enhance the confinement of the transverse mode profilewithin the active medium as its distance relative to the active mediumdecreases. By way of another example, the plunger can include adielectric portion that can lower the confinement of the transverse modeprofile within the active medium (e.g., by causing some of the radiationto “leak” into the plunger) as its distance relative to the activemedium decreases.

In another aspect, a frequency stabilized wire laser is disclosed thatincludes an active medium for generating laser radiation at a singlelasing mode. The active medium includes at least one transversedimension that is at least about 5 times, and in some cases at leastabout 10 times, smaller than the wavelength of the radiation within theactive medium. Hence, a fraction of the laser radiation propagates as anevanescent propagating wave outside the active medium. The wire laserfurther includes a spectrometer that receives a portion of the radiationgenerated in the active medium and measures the frequency of theradiation. A processor in communication with the spectrometer comparesthe measured frequency with a predefined value. If the processor detectsa deviation between the measured frequency and the predefined value thatexceeds a predefined threshold, it sends a signal to a frequencystabilization mechanism for adjusting the frequency towards thepredefined value. The frequency stabilization mechanism includes atuning element external to the active medium that is movable relative tothe active medium for interacting with a propagating evanescent wave soas to adjust a transverse laser radiation profile, thereby adjusting thefrequency towards the predefined value.

Further understanding of the invention can be obtained by reference tothe following detailed description in conjunction with the associateddrawings, which are described briefly below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically depicts a frequency tunable wire laser accordingto an embodiment of the invention; illustrating an active medium as wellas a frequency tuning mechanism;

FIG. 1B shows another schematic view of the tunable wire laser of FIG.1A, illustrating a bonding pad coupled to the active medium of the wirelaser;

FIG. 1C shows another schematic view of the tunable wire laser of FIG.1A, illustrating guide rails for moving a tuning element of the tuningmechanism;

FIG. 1D is another schematic view of the guide rails illustrated in FIG.1C;

FIG. 2A schematically shows a transverse profile of the electric fieldassociated with a laser radiation mode of the wire laser of FIG. 1A inabsence of the tuning mechanism;

FIG. 2B schematically depicts an exemplary implementation of the wirelaser of FIG. 1A in which a surface of the tuning element includes athin layer of SiN;

FIG. 3A schematically shows a transverse electric field profile of laserradiation within an active medium of a wire laser with a metal frequencytuning element a distance away from the active medium, illustrating thata portion of the transverse electric field extends outside the activemedium;

FIG. 3B schematically shows the metal tuning mechanism of FIG. 3A closerto the active medium so as to enhance the confinement of the transversemode of the laser radiation in the active medium, thereby increasing thelaser frequency;

FIG. 4A schematically shows a tunable wire laser according to anembodiment of the invention having an active medium and a frequencytuning element movable laterally relative to the active medium, wherethe tuning element is formed of a semiconductor material;

FIG. 4B schematically shows the transverse mode profile of the electricfield associated with a laser radiation mode of the wire laser of FIG.4A with the tuning element a distance from the active medium,illustrating that a fraction of the transverse electric field extendsoutside the active medium;

FIG. 4C schematically shows the semiconductor tuning mechanism of FIG.4A at a position closer to the active medium such that it extracts alarger fraction of the transverse mode outside the active medium,thereby decreasing the laser radiation frequency;

FIG. 5A graphically illustrates a conventional method for frequencytuning by changing a longitudinal wave vector of radiation within anactive medium;

FIG. 5B graphically illustrates another conventional method forfrequency tuning by changing the dielectric constant of the lasingmedium;

FIG. 5C graphically illustrates a method for frequency tuning accordingto the teachings of the invention in which the frequency is changed bydirectly changing a transverse wave vector of the radiation within anactive medium;

FIG. 6A schematically depicts a frequency tunable quantum cascadeterahertz wire laser according to an embodiment of the invention;

FIG. 6B is a cross-sectional view of the active medium of the laser ofFIG. 6A, illustrating the heterostructure composition of one of thelasing modules of the active medium;

FIG. 7A schematically shows a MEMS-based tuning mechanism suitable foruse in a tunable wire laser according to the teachings of the inventionand a device wafer to which the MEMS-based mechanism can be coupled;

FIG. 7B schematically depicts two spring stages of the MEMS-based tuningmechanism of FIG. 7A;

FIG. 7C schematically illustrates that the MEMS-based tuning mechanismof FIG. 7A can be aligned relative to the device wafer by engaging twopoles on the wafer with two openings in the actuator;

FIG. 7D schematically shows the MEMS-based tuning mechanism and thedevice wafer of FIG. 7A in an assembled configuration;

FIG. 8A schematically illustrates mechanical simulation results for adouble-fold beam implementation of the second stage of the MEMS-basedtuning mechanism of FIG. 7A;

FIG. 8B schematically illustrates simulation results of von Mises stressin the MEMS-based tuning mechanism of FIG. 7A;

FIG. 8C schematically illustrates an enlarged view of FIG. 8B showingthe stress concentration near the corners of the tuning structure;

FIGS. 9A-9F schematically depict various steps in an exemplary processfor fabricating the MEMS-based tuning mechanism of FIG. 7A;

FIG. 10A schematically shows a MEMS-based tuning mechanism according toanother embodiment, which includes an integrated comb-drive actuator;

FIG. 10B schematically shows theoretically calculated electric fieldlines for two positions of an engaged pair of fingers in one unit cellof the comb-drive actuator of FIG. 10A;

FIG. 10C schematically shows a perspective view of an engaged pair offingers in one unit cell of the comb-drive actuator of FIG. 10A;

FIG. 11 schematically depicts a tunable wire laser according to anembodiment of the invention in which the output of an active medium iscoupled to an amplifier;

FIG. 12 schematically shows another embodiment of the tunable wire laserof FIG. 11 in which a hemispherical lens is coupled to an output facetof the amplifier;

FIG. 13A schematically depicts a tunable semiconductor incoherentradiation source according to an embodiment of the invention;

FIG. 13B schematically shows a frequency stabilized wire laser accordingto an embodiment of the invention;

FIG. 14A schematically illustrates a prototype tunable wire laseraccording to the teachings of the invention in which a differentialmicrometer is used to push the long end of a lever, the short end ofwhich in turn pushes a plunger towards an active medium to change thefrequency of the laser radiation;

FIG. 14B depicts an experimental three-dimensional device mount andmechanical module utilized in the prototype device of FIG. 14A;

FIG. 14C schematically illustrates the device region in FIG. 14B,showing the plunger lying on top of guide rails, ready to be actuated bythe shaft of a linear bearing;

FIG. 14D is an image of an assembled prototype frequency tunableterahertz wire laser according to the teachings of the invention havinga silicon plunger as a tuning element;

FIG. 15A schematically depicts a terahertz wire laser according to anembodiment of the invention having a corrugated DFB grating and asemiconductor tuning plunger extracting the transverse mode of the laserradiation away from the corrugated surface of the wire laser, therebydecreasing the laser frequency;

FIG. 15B schematically depicts a terahertz wire laser according toanother embodiment of the invention having a corrugated DFB grating anda metal tuning plunger pushing the transverse mode of the laserradiation toward the corrugated surface of the wire laser, therebyincreasing the laser frequency;

FIG. 16 graphically illustrates experimental results in which the toppanel ‘a’ shows threshold current densities of a prototype terahertzwire laser according to the teachings of the invention at differentfrequencies, illustrating a moderate increase in the current density asa tuning plunger is pushed towards the laser ridge, and the bottom panel‘b’ shows broadband tuning of the laser over a range of about 137 GHz bychanging a transverse mode profile of the laser radiation within thelaser's active medium;

FIG. 17A graphically illustrates experimental results of continuousblueshift tuning of a prototype terahertz wire laser according to theteachings of the invention with a gold plunger over a range of 67 GHz;and

FIG. 17B graphically illustrates experimental results of continuousredshift tuning of a prototype terahertz wire laser according to theteachings of the invention with a gold plunger over a range of 87 GHz.

DETAILED DESCRIPTION

The present invention is generally directed to frequency tunablesolid-state radiation-generating devices, such as lasers, whose activemedium has a size in at least one transverse dimension (e.g., its width)that is much smaller than the wavelength of radiation generated withinthe active medium. For example, the size in the transverse dimension canbe at least about 5 times, or in some cases at least about 10 times,less than the radiation wavelength in the active medium. In suchdevices, a fraction of radiation travels as an evanescent propagatingwave outside the active medium. It has been discovered that in suchdevices the radiation frequency can be tuned by the interaction of atuning mechanism with the propagating evanescent wave so as to changethe transverse mode profile of the radiation in the active medium. Forexample, radiation frequency can be changed by adjusting the amount ofradiation energy carried by the propagating evanescent wave. The term“wire laser” as used herein refers to a semiconductor laser having aactive medium (herein also referred to as the gain medium or the lasingmedium) whose size in at least one transverse dimension, e.g., along adirection orthogonal to the direction of propagating laser radiation, isat least about 5 times, and in some cases at least about 10 times, lessthan the wavelength of the laser radiation in the active medium.

FIGS. 1A and 1B schematically depict a frequency tunable semiconductorwire laser 1 according to an embodiment of the invention, which includesa lasing medium 10 (herein also referred to as an active medium or again medium) extending from a proximal end (PE) to a distal end (DE)along an axial (longitudinal) direction (herein also referred to as thez-direction). In this exemplary embodiment, the lasing medium 10 is inthe form of a ridge waveguide formed on a underlying substrate 12. Inthis embodiment, the ridge waveguide has a rectangular transversecross-section characterized by a width (w) along one transversedirection (herein also referred to as the y-direction) and a height (h)along another transverse direction (herein also referred to as thex-direction). The lasing medium 10 is configured to generate laserradiation at a desired frequency. For example, in some cases, the lasingmedium 10 is configured to generate radiation in the terahertz or theinfrared portion of the electromagnetic spectrum—though other frequencyranges (e.g., the visible portion of the electromagnetic spectrum) canalso be utilized.

The lasing medium 10 includes a back facet 14 and a front facet 16 thatare cleaved and configured, e.g., via partial or complete metallization,in a manner known in the art to provide a Fabry-Perot cavity fordefining longitudinal modes of the laser radiation. The term“longitudinal mode” as used herein refers to a distribution (variation)of the laser radiation field (electric and/or magnetic field) along thepropagation direction of the laser radiation (in this case, along thez-direction). In the following discussion, the wavevector of the laserradiation along the longitudinal direction is referred to as (k_(z)).

In this exemplary embodiment, the lasing medium 10 includes afirst-order distributed feedback (DFB) grating to ensure single modeoperation of the laser. More specifically, the lasing medium includes anasymmetric DFB corrugation structure 20, where the corrugation is formedon a side surface 18 of the lasing medium 10. The opposed side surface22 is flat. In this implementation, the corrugation is in the form of asinusoidal modulation to ensure a smooth side wall 18. In manyembodiments, in the longitudinal direction under resonance condition thefollowing relation holds:

${k_{z} = \frac{\pi}{\Lambda}},$

where A is the grating period, resulting in two standing-wave solutionseach at a different frequency. In other words, the DFB grating resultsin a bandgap separating a lower band-edge mode from an upper band-edgemode. The position of the front facet 16 relative to the DFB grating canbe designed to impart a lower radiation loss to one of these lasermodes, thereby selecting that laser mode for a single-mode operation. Byway of example, this can be achieved by placing the front facet at thewidest section of the corrugated waveguide.

In this exemplary embodiment, in the upper band-edge mode the maximumlaser radiation intensity is localized at the narrow part of thegrating. This in turn results in a larger value for the transverse wavevector of the laser radiation, which can enhance the frequency tuningrange, as discussed in more detail below.

In some embodiments, the corrugation of the DFB grating can formed,e.g., by employing contact lithography and dry etching.

With reference to FIG. 1B, a bonding pad 24 is electrically coupled tothe rear facet 14 of the lasing medium, which can be metalized, to allowapplication of a voltage (e.g., a dc voltage) to, or the injection of acurrent into, the lasing medium.

The width (w) of the lasing medium 10 is significantly smaller than thewavelength (λ) of the laser radiation within the lasing medium 10. Thatis,

$\frac{w}{\lambda}1.$

For example, the width (w) is at least about 5 times, or at least about10 times, or at least about 20 times, smaller than the wavelength (λ).Consequently, a fraction of a laser radiation mode travels outside thesolid core of the lasing medium 10 as a propagating evanescent mode.More specifically, a portion of a transverse mode of the laser radiationcan extend outside the lasing medium as a propagating evanescent mode.In some embodiments, as much as about 50% of the laser radiation energycan propagate as a propagating evanescent mode. The term “transversemode” as used herein refers to a distribution (variation) of the laserradiation field (electric and/or magnetic field) along a directiondifferent from the propagation direction of the laser radiation (e.g.,along a direction orthogonal to the propagation direction, such as they-direction in this embodiment).

By way of example, FIG. 2A schematically shows the profile of theelectric field associated with a transverse laser radiation mode of thewire laser in a plane perpendicular to the laser propagation direction(e.g., along the x-direction in this embodiment) in absence of afrequency tuning mechanism 30. A portion of the electric field, andhence a portion of the laser radiation energy, penetrates outside thelasing medium 10 and travels along the longitudinal direction as anevanescent propagating mode. As discussed in more detail below, thetuning mechanism 30 is configured to interact with this evanescentpropagating mode to tune the frequency of the laser radiation.

More specifically, with reference to FIGS. 1A, 1B, 1C, and 1D, thetuning mechanism 30 can include a frequency tuning element formed, e.g.,of a metal or a semiconductor material, that is movable relative to thelasing medium 10. In this exemplary embodiment, the tuning element isconfigured to move along a plurality of guiding rails 34 formed in theunderlying substrate 12 in a direction substantially orthogonal to theflat side face 22 of the lasing medium 10. In this exemplaryimplementation, the tuning element is in the form of a plunger 32 thatis coupled to an actuator 33, e.g., a piezoelectric element, adifferential micrometer or a combination of the two or any othersuitable device, which causes it to move relative to the flat side face14. By of way of example, the actuator 33 can be configured to cause aback-and-forth movement of the plunger 32 about an equilibrium positionsuch that the plunger 32 can be moved closer to, or farther away from,the lasing medium 10 so as to modulate the interaction of the plunger 32with the evanescent propagating laser radiation, thereby adjusting thelaser frequency as discussed in more detail below. A variety ofactuators including, without limitation, mechanical, electrical orelectromechanical actuators can be employed.

With continued reference to FIGS. 1A-1D, the lengths of guide rails 34can be configured to modulate the distance between the plunger 32 andthe lasing medium 10 such that the plunger 32 can effectively interactwith the evanescent propagating radiation as it approaches the lasingmedium 10 while preventing physical contact between the plunger 32 andthe lasing medium 10. By way of example, the side wall of the plunger 32facing the lasing medium 10 (e.g., side wall 36 in this embodiment) canbe defined, e.g., by using dry etching, to include slots (not shown)matching the guide rails 34 with stoppers at the end of the guide rails34 in proximity to the lasing medium 10 (not shown) to facilitate themovement of the plunger 32 along the rails 34 while inhibiting itsphysical contact with the lasing medium 10. In some embodiments, theminimum distance between the plunger 32 and the lasing medium 10, e.g.,the flat side surface 14 in this embodiment, can be, e.g., in a range ofabout 0.5 microns to about 30 microns.

The plunger 32 can be formed of a variety of materials. By way ofexample, in some embodiments, the plunger 32 is formed of a metal, e.g.,gold, silver, copper, or aluminum, while in other embodiments, theplunger 32 is formed of a dielectric material, e.g., silicon, germanium,compound semiconductors, or plastics. With reference to FIG. 2B, in someembodiments, a face 35 of a metal plunger 33 opposed to a side face ofthe lasing medium, e.g., the flat side face 22, is covered with a thininsulating layer 37, e.g., a thin layer of SiN, to prevent shorting theplunger 33 with the device. By way of example, the thickness of theinsulating layer 37 can be in a range of about 0.1 microns to about 0.5microns.

With reference to FIGS. 3A and 3B, in the embodiments in which theplunger 32 is formed of a metal, as the plunger 32 approaches the lasingmedium 10 it enhances the confinement of the transverse mode profile ofthe laser radiation within the lasing medium, thereby causing ablue-shift of the laser radiation (i.e., an increase in the laserfrequency). Further, as the metal plunger 32 approaches the lasingmedium 10, it pushes the transverse mode towards the DFB corrugation,thereby increasing its coupling with the grating as well as enhancingthe bandgap of the grating. The increase in the bandgap accentuates theblueshift tuning as the plunger 32 approaches the lasing medium 10.

With reference to FIGS. 4A, 4B and 4C, in other embodiments in which aplunger 32′ is formed of a semiconductor material, such as silicon, asthe plunger 32′ approaches the lasing medium 10, it enhances thefraction of the laser radiation in the propagating evanescent mode. Inother words, the interaction of the silicon plunger 32′ with thepropagating evanescent wave further increases the fraction of the laserenergy propagating as an evanescent wave outside the lasing medium,thereby causing a red-shift of the laser radiation (i.e., a decrease inthe laser frequency). Further, as the semiconductor plunger 32′approaches the lasing medium 10, it extracts the transverse mode awayfrom the DFB corrugation, thereby lowering the bandgap of the grating.The lowering of the bandgap in turn enhances the redshift tuning of thelaser frequency as the silicon plunger 32′ approaches the lasing medium10. In this embodiment, the silicon plunger 32′ includes a hollow cavity42, which defines a width (wp) of the plunger to be small enough so asto ensure a sufficient mode overlap with the lasing medium to achievelasing. By way of example, in some embodiments, the width (wp) of thesemiconductor plunger can be less than about 20 microns, e.g., it can bein a range of about 10 microns to about 20 microns.

In some embodiments, the tuning mechanism allows tuning the laserfrequency over a range that is at least about 30% of a center frequency.In many embodiments, a side of the plunger, e.g., side surface 36 of theplunger, is aligned parallel to a side of the lasing medium relative towhich it moves, e.g., the flat side 32 of the lasing medium 10, with analignment accuracy of at least about 1 degree to maximize the frequencytuning range of the laser. Such alignment accuracy can be achieved, forexample, by employing alignment posts during fabrication or implementingthe plunger as two spring stages where the second spring stage isstiffer, or other suitable techniques.

The above tuning of the wire laser can be achieved without causing asubstantial change, or in some cases any change, in the longitudinalmode profile of the laser radiation, e.g., via external tuning elementsand/or a change in the temperature of the lasing medium. Alternatively,in some embodiments, frequency tuning by adjusting a transverse modeprofile of a wire laser can be combined with other types of frequencytuning (e.g., a change in the longitudinal mode profile and/or thedielectric constant of the lasing medium).

Without being limited to any particular theory or implementation, thefrequency tuning of a wire laser by changing its transverse mode profilevia interaction with a propagating evanescent wave can be perhaps betterunderstood by considering that in a uniform gain medium the transversewavevector (k_(⊥)), the longitudinal wavevector (k_(z)) and the angularfrequency (ω) of the laser radiation are related by the followingrelation:

k _(z) ² +k _(⊥) ²=ω²μ₀∈  Eq. (1),

wherein,

k_(z) denotes the longitudinal wave vector,

k_(⊥) denotes the transverse wave vector, which is defined as:

k_(⊥)=√{square root over (k _(x) ² +k _(y) ²)}, where k_(x) and k_(y)denote, respectively, wave vectors along the x and y directions in aplane perpendicular to z direction (i.e., the propagation direction ofthe laser radiation),

μ denotes magnetic permeability of the lasing medium,

∈ denotes dielectric constant of the lasing medium, and

ω denotes angular frequency of the laser radiation, which in turn isdefined as:

ω=2πf, where f denotes the laser frequency.

The above Equation (1) shows that a laser frequency (f) can be tuned bychanging one or more of the following three parameters: k_(z), k_(⊥),and ∈. In conventional tunable lasers, only k_(z) and/or ∈ are changedfor tuning the laser. In contrast, the frequency tunable wire lasersaccording to the teachings of the invention rely on changing k_(⊥) toachieve frequency tuning.

By way of further illustration, FIG. 5A schematically depicts that inmost commonly used conventional frequency tuning mechanisms, both k_(⊥)and ∈ are fixed and the tuning of the laser frequency is achieved bychanging k_(z), e.g., by changing the cavity length of a Fabry-Pérotlaser or by changing the center wavelength of distributed Braggreflectors (DBRs), or changing the angle of an external cavity grating.With reference to FIG. 5B, in another conventional mechanism, frequencytuning is achieved by changing the dielectric constant of the lasingmedium (∈), e.g., by temperature tuning or electrical bias. In contrast,with reference to FIG. 5C, in a tuning mechanism according to manyembodiments of the invention, the frequency of a wire laser is tuned bydirectly changing the transverse wave vector k_(⊥).

As discussed above, the teachings of the invention can be employed toprovide frequency tuning of a variety of wire lasers adapted to generateradiation in different portions of the electromagnetic spectrum.

By way of example, FIG. 6A schematically depicts a semiconductor wirelaser 100 according to an embodiment of the invention that includes anactive lasing medium 102 for generating radiation in a frequency rangeof about 1 THz to about 10 THz. In this exemplary embodiment, thesemiconductor laser 100 comprises a quantum cascade laser in which thelasing medium 102 includes an active lasing region that is formed as aheterostructure on an underlying substrate 106. More specifically, withreference to FIG. 6B, in this exemplary implementation the active lasingregion 102, which can have a thickness in a range of about 3 microns toabout 10 microns, includes a plurality of cascaded nominally identicalrepeat lasing modules 104 that are coupled in series. The number oflasing modules 104 can range, for example, from about 100 to about 200.

In this exemplary embodiment, each lasing module 104 is formed of aGaAs/Al_(0.15)Ga_(0.85)As heterostructure. For example, each lasingmodule 104, which can have an approximate thickness of about 600angstroms, is formed as a stack of alternating Al_(0.15)Ga_(0.85)As andGaAs layers having the illustrated thicknesses. The heterostructure ofeach lasing module 104 provides quantum wells that collectively generatelasing and relaxation states.

The frequency tunable terahertz quantum cascade laser 100 furtherincludes a double metal waveguide formed of metal layers 108 and 110that confine the laser mode within the lasing medium in one transversedirection (labeled herein as the x-direction). Further details regardingthe structure of the lasing medium as well as the double metal waveguidecan be found in U.S. Pat. No. 7,548,566 entitled “Terahertz lasers andamplifiers based on resonant optical phonon scattering to achievepopulation inversion,” which is herein incorporated by reference in itsentirety.

As discussed in more detail below, the laser mode in the othertransverse direction (labeled herein as the y-direction) is not entirelyconfined within the lasing medium 102, but rather extends beyond thelasing medium 102 as a propagating evanescent wave. More specifically,the width (w) of the lasing medium 102 is significantly less than awavelength of the laser radiation (e.g., by a factor of at least about 5or at least about 10) in the lasing medium 102, thus resulting in asubstantial fraction of the laser radiation propagating as an evanescentwave.

Similar to the above frequency tunable laser 10, the tunable terahertzlaser 100 illustrated in FIG. 6A also includes a frequency tuningmechanism 112 for interacting with the evanescent propagating wave ofthe laser mode in order to tune the laser frequency in a mannerdiscussed above. For example, as discussed in detail above, thefrequency tuning mechanism 112 can include a metal or a semiconductorplunger 114 movable relative to the lasing medium 102 that can modulatethe confinement of the transverse laser mode (in this case, thetransverse mode along the x-direction) in the lasing medium 102, therebytuning the laser frequency.

Further, in this exemplary implementation, the lasing medium 102includes a flat side 116 and a corrugated side surface 118 so as toprovide an asymmetric DFB for achieving single mode operation, andfurther facilitating the frequency tuning of the laser 100 in a mannerdiscussed above.

A variety of frequency tuning mechanisms can be employed in tunable wirelasers according to the teachings of the invention. By way of example,in some implementations, the tunable mechanism can be formed as amicroelectromechanical systems (MEMS). By way of example, in someembodiments, the frequency tuning mechanism can include a MEMS-basedplunger that is suspended over the device wafer, e.g., by a distance ofabout 1 micron, so as to minimize, and preferably eliminate, frictionbetween the plunger and the device substrate.

By way of example, FIGS. 7A and 7B schematically show a MEMS-basedplunger 130 according to an embodiment of the invention as well as adevice substrate 132 on which a wire laser 134 is formed and to whichthe MEMS-based plunger 130 can be coupled, as discussed in more detailbelow. The plunger 130 includes a first stage 136 and a second stage138. The first stage 136 includes a three-fold spring 140 to achieve alarger displacement than the second stage 138 (e.g., the first stage canachieve a displacement of about 30 microns while the second stageachieves a displacement of about 20 microns) and to isolate anymisalignment between an external actuator (not shown), such as adifferential micrometer, and the second stage 138. The second stage 138in turn employs a double-folded beam design that provides a stifferstructure in the transverse direction. In this implementation, thesecond stage includes a side surface 142 that is coated with a metal,e.g., gold, that can interact with a propagating evanescent wave of thelaser radiation. In this implementation, the first and second stages136, 138 are formed of silicon. A sidewall 142 of the plunger 130 isformed of a top layer of silicon, a relatively thin layer of SiO₂,followed by a relatively thick layer of silicon coated with a layer ofgold between the silicon and the flat side of the wire laser 134. Thesubstrate 132 is formed of GaAs coated with a layer of gold between theMEMS-based plunger 130 and the GaAs.

As shown in FIG. 7C, the plunger 130 includes two openings 144 and 146at a bottom surface 148 thereof that are configured for engagement withtwo poles 150 and 152, formed, e.g., of gold, on the device substrate inorder to assemble the plunger with the device substrate.

FIG. 7D schematically depicts an assembled device in which theMEMS-based plunger 130 can be moved relative to a side surface of thewire laser 134 (e.g., the flat surface of a DFB laser ridge) to tune thefrequency of the laser radiation. For example, an external actuator (notshown), such as a mechanical drive (e.g., a differential micrometer),can apply a force, e.g., a compressive force, to the first stage 136, tocause it to move, e.g., with a moving range of about 25 microns. Thefirst stage 136 in turn applies a force to the second stage 138 so as tomove the second stage 138, and consequently the gold-coated side wall142, from its free-standing position along a transverse directiontowards a side surface of the laser 134 (e.g., the flat surface of a DFBlaser ridge). Conversely, the lowering of the compressive force (e.g.,adjusting a micrometer in a reverse direction) can cause relaxation ofthe spring structures so as to retrieve the plunger surface 142 back toits free-standing position. In this manner, the gold-coated side wall ofthe plunger 130 can interact with a transverse propagating evanescentfield of the laser radiation for tuning the laser frequency. In thismanner, the frequency tuning of the laser 134 can be made reversibly.

In some implementations, at a free-standing position, the plungersurface 142 can be at a distance in a range of about 15 microns to about20 microns from the side surface of the laser 134. In some cases, tuningof the laser frequency over the desired range can be achieved bychanging the distance between the plunger surface 142 and the laser sidesurface over a range of about 5 to about 10 microns provided, e.g., thatthe plunger surface 142 is parallel with the DFB laser ridge to withinabout 1 degree. The use of the two stages 136 and 138 advantageouslyallows achieving such a high precision and a large moving range. Inparticular, as noted above, the second stage 138 is stiffer and henceits alignment with the laser side surface can be more readily retained.

In some embodiments, the mechanical design of the flexure stages 136 and138 can be facilitated by using finite-element (FEM) simulation using,e.g., a commercial software package such as FEMLAB 3.5 marketed byComsol Inc. of Burlington Mass., U.S.A. By way of example, thesimulation can be employed to choose the proper thickness and length ofthe flexure stages such that the maximum stress in the structure wouldbe much smaller than the yield stress, which was chosen to be about 250MPa. In some embodiments, in the fabrication process, a series ofdifferent widths can be tried in order to determine the optimal width.In MEMS flexure design, sharp edges in a structure are generally avoidedas sharp edges can lead to stress concentration and unpredictablefabrication results. Hence, in many implementations, the corners of theflexure structures are rounded, e.g., with a filler having a 20-μmradius.

By way of illustration, FIGS. 8A-8C show exemplary simulation resultsfor a double-folded beam implementation of the second flexure stage 138having a width of 20 microns and a length of 750 microns. FIG. 8A showsthe deformed shape of the beam structure, which can achieve adisplacement range of about 20 microns. FIG. 8B shows the calculated vonMises stress in the structure, and FIG. 8C shows enlarged view of thestress concentration near the corners.

With reference to FIGS. 9A-9C, in one method for fabricating theMEMS-based plunger, a Silicon-On-Insulator (SOI) wafer 160 is provided,which includes a silicon (Si) device side 162, a Si handle side 164 anda silicon oxide layer 166 that separates the device side 162 from thehandle side 164. By way of example, the device side 162 can have athickness of about 50 microns and the handle side 164 can have athickness of about 400 microns. Initially, with a photoresist as a mask,a trench 168 can be formed in the silicon device layer 162, as shown inFIG. 9B, by employing continuous dry-etching to avoid contact betweenthe plunger 160 and the device wafer (i.e., the wafer on which the wirelaser is located) when the plunger 160 is assembled to the device wafer.

Subsequently, an SiO₂ layer, e.g., one having a thickness of about 2microns, can be deposited on each side of the wafer (i.e., the deviceside 162 and the handle side 164) and annealed, e.g., at a temperatureof about 1000 C. The SiO₂ layer 166 can be patterned by standardlithography and BOE (Buffered Oxide Etchant) wet etching. By way ofillustration, FIG. 9C shows exemplary patterned SiO₂ layer 170.

Subsequently, as illustrated in FIG. 9D, a thick photoresist layer 172,e.g., one having a thickness of about 1 micron, can be spun on thehandle side 164 of the wafer and patterned. The combined masks of thephotoresist 172 and the SiO₂ 170 can be used to define the stepstructure in the handle side 164 of the wafer. The wafer can then beloaded into an etcher (e.g., an inductive coupled plasma-reactive ionetching device, such as STS ICP-RIE etcher) to be etched, e.g., for 200microns, with a calibrated recipe of DRIE (deep RIE). This can befollowed by stripping the photoresist 172 as shown in FIGS. 9E and 9F,and etching the handle side 164 of the wafer again for 200 microns untilthe etched portions reach the embedded SiO₂ 166. DRIE can then be usedto define the desired pattern on the device side with the SiO₂ 166 asthe hard mask. Next, buffered oxide etchant and critical point releasingapparatus can be used to release the embedded SiO₂ 166 to prevent themovable structure from sticking to the bottom substrate. Finally, theside of the plunger that will face the DFB laser ridge upon assembly ofthe plunger with the device wafer can be coated with a metal, e.g.,Ti/Au, by employing known techniques, such as e-beam evaporation.

The MEMS structure can be assembled to the device wafer and aligned withthe laser. For example, referring again to FIG. 7C, the alignment poles150, 152 on the device wafer and the matching holes 144, 146 on the MEMSwafer can be utilized to align the plunger 130 with the laser 134 and toretain the two wafers fixed relative to each other. The free movableplunger 130 can then be moved by an external actuator, e.g., adifferential micrometer, and pulled back by the MEMS springs when themicrometer is dialed back.

In some embodiments, a MEMS plunger with integrated comb-drive actuatorcan be employed for electronically tuning the frequency of a wire laseraccording to the teachings of the invention. By way of example, FIG. 10Aschematically depicts an example of such a plunger 200 that includes acomb-drive actuator 202 having movable set (rotor) 206 comb fingers thatare engaged with a stationary set (stator) 208 of comb fingers. A groundplate (not shown) is located under the comb fingers, which is normallyconnected to the same potential as the rotor 206 in order to preventapplication of electrostatic pull-down forces to the substrate. Theelectrostatic force of the comb-drive 202 can provide the actuationforce for moving the movable spring stages of the plunger 200.

By way of further illustration, FIGS. 10B and 10C schematically show anengaged pair of fingers 210, 212 in one unit cell of the comb-driveactuator 202, which can be analyzed as a parallel-plate capacitor. Insome implementations in which voltage control is employed, the lateralelectrostatic force in the y-direction, which is equal to the negativederivative of the electrostatic coenergy with respect to they-direction, is attractive. This force acts on the rotor 206, which isdirectly connected to the spring structure that moves the plunger 200.By way of illustration in FIG. 10C, a simple model that neglects thefringing fields can be employed to estimate the required number offingers in the comb-drive actuator 202. For example, for a maximumdisplacement of about 20 microns, a comb height of about 50 microns, aseparation distance d of about 6 microns, and a drive voltage of about100 volts, the required number of fingers can be estimated to be about300.

In some embodiments, the output radiation generated by a tunable wirelaser according to the teachings of the invention can be coupled into anamplifier, e.g., in a master-oscillator-power-amplifier configuration,so as to amplify the radiation. In some cases, the amplification stagecan also improve the beam pattern of the laser radiation.

By way of example, FIG. 11 schematically illustrates a laser device 201according to such an embodiment of the invention that includes a tunablewire laser 230 according to the teachings of the invention, such as theabove THz tunable wire laser, whose output is optically coupled to apower amplifier 232 (e.g., in this implementation an input facet 234 ofthe amplifier 232 is attached to an output facet 236 of the wire laser).In this embodiments, the wire laser 230 has a DFB grating 238 and theamplifier 232 has a tapered configuration with its output facet 240anti-reflection (AR) coated and tilted relative to the DFB ridge 238 toprevent direct reflection of radiation back to the DFB oscillator.Further, to avoid oscillations in the amplifier 232, an absorbing layer242 can be placed on a top surface of the amplifier 232 along itsslanted edge. The absorbing layer 242 can dampen any residual reflectionfrom the tilted output facet 240 of the amplifier 232 to prevent lasingoscillation in the amplifier 232. By way of example, the absorbing layer242 can be a thin heavily-doped GaAs layer having a thickness of, e.g.,about 0.1 microns uncovered with metal so the electric field will not beshorted by it.

With reference to FIG. 12, in some implementations, a lens 260, e.g., ahyperhemispherical lens formed of silicon, can be optically coupled,e.g., attached, to an output facet 262 of an amplifier 264 to increasethe radiation output power level of the wire laser 266 and to reduce thedivergence of the output beam. Further details regarding examples of thelens 260 and its use can be found in published U.S. Patent ApplicationNo. 2010/0002739 entitled “Lens Coupled Quantum Cascade Laser,” which isherein incorporated by reference in its entirety.

The teachings of the invention are not limited to coherent radiationsources, but rather can be employed to fabricate frequency tunablesemiconductor incoherent radiation sources. By way of example, FIG. 13Aschematically depicts a tunable semiconductor incoherent radiationsource 280, such as a frequency tunable amplified stimulation emission(ASE) device, according to an embodiment of the invention that includesa semiconductor active medium 282 that is configured for generatingradiation in a desired frequency range, e.g., in a range of about 1 THzto about 10 THz, or the visible range of the electromagnetic spectrum.Input and output facets 284 and 286 of the active medium can be cleavedand otherwise configured to prevent lasing oscillation in the activemedium. For example, the output facet 286 can be anti-reflection coatedto minimize reflection feedback from that facet into the active medium.

The active medium 282 has a width (w) that is much smaller than thewavelength of radiation (λ) in the active medium. For example, the width(w) can be at least about 5 times, or at least about 10 times, less thanthe wavelength of the radiation within the active medium. Hence, afraction of radiation propagates as an evanescent propagating waveoutside the active medium 282. A frequency tuning mechanism 283 externalto the active medium 282, such as the mechanisms discussed above,interacts with the propagating evanescent wave so as to tune thefrequency of the radiation. More specifically, an actuator 285 can beemployed to change the distance between a tuning element 287 of thetuning mechanism and a side surface of the active medium so as to changethe transverse mode profile of the radiation, thereby adjusting theradiation frequency.

In other aspects, the invention provides frequency stabilizedradiation-generating semiconductor sources in which the frequency isstabilized via a feedback loop that adjusts a transverse mode profile ofradiation within an active medium. By way of example, FIG. 13Bschematically depicts a frequency stabilized wire laser 300 according toan embodiment of the invention that includes an active medium 400 forgenerating laser radiation with a frequency in a desired range, e.g., ina range of about 1 THz to about 10 THz. In this embodiment, the activemedium 400 includes a DFB structure 414 that ensures single mode lasing.The active medium 400 has a width (w) that is significantly less thanthe wavelength of radiation within the medium 400 (e.g., at least about5 times or at least about 10 times less than the wavelength). Hence, afraction of the laser radiation propagates outside the active medium 400as a propagating evanescent wave.

The laser radiation leaves the active medium 400 through an output facet402 thereof. A beam splitter 404 directs a small portion of the laserradiation to a spectrometer 406 that determines the frequency of thelaser radiation. The spectrometer 406 communicates the measuredfrequency to a processor 408, which in turn compares the measuredfrequency with a predefined value. If the processor 408 determines adeviation between the measured and the predefined frequencies, it sendsa signal to an actuator 410 coupled to a tuning element 412 to cause themovement of the tuning element 412 towards or away from the activemedium 400. In this manner, the tuning element 412 can interact with apropagating evanescent wave to adjust the frequency to the predefinedvalue.

The following example is provided to further illustrate the salientfeatures of the invention. The example is provided only for illustrativepurposes and is not intended to necessarily indicate optimal structuresand results that can be obtained by practicing the teachings of theinvention.

Example

A prototype frequency tunable semiconductor quantum cascade laseroperating in the terahertz portion of the electromagnetic spectrum wasfabricated according to the above teachings of the invention. Thisprototype device included an active region (herein also referred to asthe THz gain medium) formed as a stack of lasing modules, each of whichwas formed as heterostructure of alternating layers ofAl_(0.15)Ga_(0.85)As and GaAs (the layer composition of one of theheterostructures is shown in FIG. 6B above). The active region wasformed as a ridge waveguide on an underlying n+GaAs substrate having athickness of about 100 microns by employing molecular beam epitaxy. Adouble-metal waveguide formed by wafer bonding and substrate removal,with one layer deposited in top and the other on the bottom of theactive region was employed for mode confinement along the height of theactive region. A Fabry-Perot cavity was formed by cleaving the proximaland distal ends of the ridge waveguide to form a back and front facet.The back facet was metalized with a coating of gold.

As illustrated in FIGS. 14A-14D, an asymmetric first-order distributedfeedback grating (DFB) was fabricated in the active region, e.g., in amanner discussed above, to ensure single mode lasing. The asymmetric DFBincluded a corrugated structure 300 formed on a side surface of theactive region. The opposed side surface 302 of the active region wasflat. The DFB corrugation 300 was in the form of sinusoidal modulation,which was chosen to provide for a smooth undulating side wall. The laserridge had an average width of about 12.5 μm, sinusoidal gratingmodulation of 3 μm, 30 periods and a grating period of Λ=13.7 μm. Theheight of the gain medium was ˜10 μm.

The front facet 304 of the laser ridge was chosen to be the widest partof the DFB grating to increase the radiation loss of the lower band-edgemode and its lasing threshold. The rear facet of the laser ridge 300 wasdefined using wet etching with a slope so that a bonding pad 306 couldbe fabricated away from the laser ridge 300 and the electrical contactto the device mount 308 could be made without interfering with themovement of a plunger 310 utilized for frequency tuning, as discussedbelow. The rear facet was metalized with gold to facilitate forming anelectrical contact with the bonding pad 306.

A number of guide rails 312 were fabricated by employing dry etchingperpendicular to the laser ridge 300 on the device mount 308 for guidingthe plunger 310, as illustrated in FIG. 14C. The length of the guiderails 312 was precisely defined to prevent the plunger 310 fromcontacting the laser ridge 300. A metal plunger 314 and silicon plunger316 were fabricated for frequency tuning of the laser (See FIGS. 15A and15B). The side wall 318, 320 of each plunger 314, 316 facing the laserridge 300 was defined using dry etching, and slots and stoppers wereformed along its base to match the guide rails 312 on the device mount308. For the silicon plunger 316, a reduced width was required, as shownin FIGS. 14D and 15A, so that the mode remained appreciably in the gainmedium to achieve lasing. This reduced width was achieved using dryetching. On both plungers 314, 316, a portion 322 overhang the laserridge, as shown in FIG. 14D. It was determined that the overhang 322 hadlittle effect on the tuning process because it was ˜7 μm higher than thetop of the laser ridge 300, where the evanescent mode is negligible.

The laser ridge 300 and the plunger 314, 316 were mounted onto a coldplate device mount 308, as illustrated in FIG. 14C. During operation,the particular plunger 314, 316 in use was pressed down onto the devicemount 308 so that it could only be pushed toward the laser ridge 300. Toovercome the friction between the plunger 314, 316 and the guide rails312, a piezoelectric-transducer (PZT) bender (P-871.140 Piezo BenderActuator marketed by PI (Physik Instrumente) of Massachusetts, U.S.A.)and a mechanical differential micrometer shown in FIG. 14B were used.The PZT bender had a continuous displacement range of about 800 ineither direction at room temperature. The micrometer had a movementresolution of about 0.5 μm. This resolution was further improved byusing a ˜4:1 lever, as also shown in FIG. 14B, resulting in a ˜140 nmstep resolution. The mechanical module was made mostly of steel with theexception of the thermal isolator, which was composed of fiber glass.

The device mount was formed from copper and was mounted in a vacuumcryostat during testing, as shown in FIG. 14A. The emitted laser lightwas collected without any optical components inside the cryostat. All ofthe spectra were measured at 5 K using a Nicolet 850 spectrometer, whichwas purged with N₂ gas, and a germanium gallium photodetector in pulsemode with a frequency of 90 kHz and a duration of 200 ns.

The design of the DFB/plunger prototype was aided with finite-element(FEM) simulations using a commercial software package (Comsol 3.2). Themetal was treated as a perfect conductor and the semiconductor wasundoped. The calculated gain threshold g_(th) therefore only reflectedradiative losses. For a bare DFB laser, writing k_(z)=n_(eff)(ω/c)=π/Λ,an effective mode index of n_(eff)≈2.86 was estimated for the lowestloss mode, which is considerably lower than the refractive index of theactive medium (n_(active)=3.6 used for calculation) and is indicative ofa large fraction of the mode propagating outside the active medium.

In the longitudinal direction under the resonance condition k_(z)=π/Λ,where Λ is the grating period, two standing-wave solutions exist andeach at a different frequency, forming a bandgap. The upper band-edgemode has the maximum intensity localized at the narrow part of thegrating, which results in a larger value of k_(⊥). Similarly, themaximum intensity of the lower band-edge mode is located at the widerpart of the grating. The front facet 304, from which the radiation iscoupled out, is open and can be defined by dry etching. By carefullypositioning the front facet 304 relative to the DFB grating 300, eitherthe lower band-edge or the upper band-edge mode will have lowerradiation loss, and will be the lasing mode in a single-mode operation.

For this experiment, the upper band-edge mode was chosen to be thelasing mode to enhance the tuning range. As the metal plunger 314 waspushed towards the laser, the mode was pushed towards the DFBcorrugation as shown in FIG. 15B, increasing its coupling with thegrating and therefore the bandgap (from ˜260 to ˜417 GHz with a gap of0.5 μm). This increase of the bandgap accentuated the blueshift tuning.Similarly, as the silicon plunger 316 was pushed towards the laser, themode was extracted away from the DFB corrugation as shown in FIG. 15Aand the bandgap shrinks (from ˜260 to ˜120 GHz with a gap of 0.5 μm),effectively enhancing the redshift tuning. Based on this, the frontfacet 304 was chosen to be at the widest part of the DFB grating, asshown in FIGS. 15A and 15B, which increased the radiation loss of thelower band-edge mode and its lasing threshold.

The results obtained with this prototype, including both redshift andblueshift, from a single device are shown in FIG. 16. Using the plungers314, 316 described above, a redshift tuning of about 57 GHz and ablueshift tuning of about 80 GHz were achieved. In combination, a totaltuning range of about 137 GHz, or ˜3.6% fractional tuning, was achieved.During the spectral measurements, both the device bias and thetemperature were kept constant, so tuning due to Stark shift and/ortemperature was negligible. In fact, the frequency shift of the DFBdevice with bias, with the plunger in a fixed position, was notmeasurable within the resolution (3.75 GHz) of the spectrometer. Hence,the frequency shift of the single lasing mode, shown in FIGURE D, can beunambiguously attributed to the tuning of k_(⊥) by the movement of theplunger.

In this prototype device, the measured power levels change with theplunger position in a complicated way, due to the changes in beampattern and in atmospheric attenuation with frequency. Also, theabsolute power was too low in this experiment to be measurable with apower meter because the assembly required the device to be placed farfrom the Dewar window and did not allow the use of a collecting opticsuch as a metallic cone to collect the highly divergent beam emittedfrom the laser facet. Thus, a more meaningful parameter forcharacterizing the laser performance is the threshold current atdifferent plunger positions. This result is shown in the upper panel inFIG. 16, which exhibits a moderate increase as either a silicon or ametal plunger is pushed towards the laser ridge. For the silicon plunger316, this increase is due to a reduction of the mode confinement factorin the gain medium. For the metal plunger 316, the increase of thelasing threshold is likely due to loss of in the metal and a thin SiNinsulation layer preventing shorting the plunger with the device.

The tuning spectra shown in FIG. 16 show some discontinuous jumps,likely due to the stick-slip effect of the plunger as it is movedrelative to the laser ridge. FIGS. 17A and 17B show clearer continuoustuning, although with smaller tuning ranges, from two measurements ondifferent tunable prototype DFB laser devices. FIG. 17A shows acontinuous blueshift tuning using a metal plunger, as expected. Theresult shown in FIG. 17B is also from a metal plunger. However, it showsa redshift tuning. This puzzling result was understood when the set-upwas inspected under a microscope at room temperature. It became clearthat the plunger had been tilted inadvertently, and its overhang washinged on the edge of the laser ridge, as illustrated in the inset inFIG. 17B. As the upper part of the metal plunger is pushed forward, theplunger is further tilted and the effective gap between the plunger andthe laser ridge increases, resulting in the observed redshift tuning.Interestingly, free from the friction problem, this scheme allows areversible tuning, as a retreat of the bender relaxes the tilt of theplunger, yielding a blueshift.

The novel tuning mechanism of laser frequency, demonstrated here bychanging k_(⊥), applies to wire lasers at any frequencies. For recentlydeveloped wire lasers at visible frequencies, a scanning probe could beused to manipulate the transverse mode profile to tune the frequency forsensing and spectroscopy at nanometer scales. For terahertz wire lasers,the use of MEMS technology allows for better control of the plunger,resulting in a finer tuning over a broader frequency range in areversible way. Such a controlled tuning also offers a mechanism forfrequency stabilization using feedback control. Finally, the generatedtunable single-mode signal from a wire laser can be fed into anintegrated terahertz amplifier to produce high-power radiation with goodbeam patterns. Further information regarding the above prototype devicescan be obtained in an article entitled “Tuning a terahertz wire laser,”which was published in Nature Photonics, vol. 3, December 2009. Thisarticle is herein incorporated by reference in its entirety.

Those having ordinary skill in the art will appreciate that variouschanges can be made to the above embodiments without departing from thescope of the invention.

What is claimed is: 1.-40. (canceled)
 41. A Terahertz amplifier,comprising an amplification region for amplifying an incoming radiationsignal having a frequency in a range of about 1 THz to about 10 THz togenerate an amplified signal, characterized by a transverse distributionof the amplified signal such that a fraction of said signal extendsoutside the amplification region as the radiation signal propagatesalong the amplification region, an input port for coupling said incomingradiation into said amplification region, an exit port for extractingsaid amplified signal from said amplification region, and a tuningelement external to said amplification region, said element beingmovable relative to said amplification region for interacting with theamplified signal extending outside the amplification region so as tochange a frequency of said amplified signal.
 42. A tunable radiationsource, comprising: an incoherent radiation source having an activemedium for generating radiation, at least a portion of said radiationpropagating along the active medium as a propagating evanescent wave;and a frequency tuning element external to said incoherent radiationsource, said frequency tuning element being movable relative to saidincoherent radiation source for interacting with the propagatingevanescent wave in order to adjust a transverse mode profile of theradiation in the active medium, thereby changing the frequency of theradiation.